I've ended up with a new problem. For some time, I thought the machine was leaking. Well, it was leaking. I'd pour the water back into the Humminguru and it would start dripping into the container. No matter what I did, it would always drip a bit. (Fortunately I have the machine on our floor with underfloor heating, but it was a real pain).
I put up with it for a couple of months, cursing the machine repeatedly. Today, I decided to have a proper look - opened the machine up, cleaned the one water pipe I could get easy access to - still the same problem. I realised though that there was always a bit of water left in the machine and now it had become more. So I tried manually stopping the machine as it started to blow dry, and starting the dry cycle again (so it drains the machine a second time). And that has worked.
So for some reason it isn't draining quickly enough. (It's leaving a good centimetre plus of water now, and takes about 10-15 seconds of draining the second time round to hear that sucking sound that means there's no water left). I've reached out to Humminguru but posting here in case anybody has any thoughts / experience on this. I reckon I've cleaned somewhere between 3000-4000 pieces of vinyl so probably no wonder it's starting to fall apart. Still got about a fifth of my collection to go though...